1. Field of the Invention
The invention relates to the field of controlling or modifying the thermal profile in the vicinity of the growth control surfaces of an EFG die within an EFG furnace.
2. Prior Art
Prior art EFG (Edge-defined Film-fed Growth) furnaces have utilized crucible covers which cover the crucible except where the die extends through the cover. It is well-known to provide a stack of flat heat shields above the crucible cover. Spacers space these heat shields from each other and from the cover. The heat shield stack changes the thermal profile above the cover from what it would be in the absence of the heat shields. It is well-known in the art to vary the number and spacing of the heat shields in the stack in an attempt to control the thermal profile at the top of the die and the thermal gradient in the first few inches above the die (the region first traversed by the newly crystallized ribbon material). It is also known to vary the vertical alignment between the induction heating coil and the crucible. These techniques are used to provide proper crystallization conditions at the liquid-solid growth interface and to provide a thermal gradient for aftergrowth annealing of the ribbon material and to prevent excessive thermal gradients which could crack the newly grown crystal. Such heat shield systems have not been fully effective in controlling the thermal profile at the top of the die. Improved thermal profile control at the die top is needed in order to grow higher quality, relatively wide ribbons, especially when growing a plurality of ribbons from a single die.